Time dependent neutron-gamma spectroscopy

ABSTRACT

An apparatus and method for estimating at least one parameter of interest of an earth formation including an estimation of the parameter of interest using a time-dependent ratio of information obtained from at least one neutron detector through the exposure of the earth formation to a radiation source, particularly a pulsed nuclear source. The apparatus includes a processor and storage subsystem with a program that, when executed, implements the method. Also, an apparatus and method for estimating at least one parameter of interest of an earth formation including an estimation of the parameter of interest using a first component and a second component of an information set obtained using a single radiation detector.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/299,191 filed on 28 Jan. 2010.

FIELD OF THE DISCLOSURE

In one aspect, this disclosure generally relates to borehole loggingmethods and apparatuses for estimating formation properties usingnuclear radiation based measurements. More particularly, this disclosurerelates to estimating one or more formation parameters of interest usinginformation obtained from a formation exposed to a pulsed neutronsource.

BACKGROUND OF THE DISCLOSURE

Oil well logging has been known for many years and provides an oil andgas well driller with information about the particular earth formationbeing drilled. In conventional oil well logging, during well drillingand/or after a well has been drilled, a radiation source and associatedradiation detectors may be conveyed into the borehole and used todetermine one or more parameters of interest of the formation. A rigidor non-rigid carrier is often used to convey the radiation source, oftenas part of a tool or set of tools, and the carrier may also providecommunication channels for sending information up to the surface. Thetool or set of tools may be configured to store information for laterretrieval.

SUMMARY OF THE DISCLOSURE

In aspects, the present disclosure is related to methods of estimating aparameter of interest of an earth formation using radiation detectedfrom a subterranean formation.

One embodiment according to the present disclosure may include a methodfor estimating at least one parameter of interest of an earth formation,comprising: estimating the at least one parameter of interest using afirst component and a second component of an information set obtainedusing a single radiation detector.

Another embodiment according to the present disclosure may include anapparatus for estimating at least one parameter of interest of an earthformation, comprising: at least one processor; a storage device; and aprogram stored on the storage device comprising instructions that, whenexecuted, cause the at least one processor to: estimate the at least oneparameter of interest using a first component and a second component ofan information set obtained using a single radiation detector.

Another embodiment according to the present disclosure may include amethod for estimating at least one parameter of interest of an earthformation, comprising: estimating at least one parameter of interestusing a time-dependent ratio based on information acquired from at leastone neutron detector and a second detector.

Another embodiment according to the present disclosure may include amethod for estimating at least one parameter of interest of an earthformation, comprising: estimating at least one parameter of interestusing a time-dependent ratio based on information acquired from at leastone neutron detector and a second detector; and combining referenceinformation relating to the earth formation with the time-dependentratio to estimate the at least one parameter of interest, wherein the atleast one parameter of interest includes at least one of: porosity,sigma, diffusion correction, and hydrogen index.

Another embodiment according to the present disclosure may include anapparatus for estimating at least one parameter of interest of an earthformation, comprising: a processor; a storage subsystem; and a programstored by the storage subsystem comprising instructions that, whenexecuted, cause the processor to: estimate a time-dependent ratio basedon information acquired from at least one neutron detector and a seconddetector.

Another embodiment according to the present disclosure may include anapparatus for estimating at least one parameter of interest of an earthformation, comprising: a processor; a storage subsystem; and a programstored by the storage subsystem comprising instructions that, whenexecuted, cause the processor to: estimate a time-dependent ratio basedon information acquired from at least one neutron detector and a seconddetector; and combine reference information relating to the earthformation with the time-dependent ratio to estimate the at least oneparameter of interest, wherein the at least one parameter of interestincludes at least one of: porosity, sigma, diffusion correction, andhydrogen index.

Examples of the more important features of the disclosure have beensummarized rather broadly in order that the detailed description thereofthat follows may be better understood and in order that thecontributions they represent to the art may be appreciated.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the present disclosure, reference shouldbe made to the following detailed description of the embodiments, takenin conjunction with the accompanying drawings, in which like elementshave been given like numerals, wherein:

FIG. 1 shows a schematic of a downhole tool deployed in a wellbore alonga wireline according to one embodiment of the present disclosure;

FIG. 2 shows a flow chart of an estimation method for one embodimentaccording to the present disclosure;

FIG. 3 shows schematic of the apparatus for implementing one embodimentof the method according to the present disclosure;

FIG. 4 shows a graphical illustration of the detector count rates andratio over time according to one embodiment according to the presentdisclosure;

FIG. 5 shows a graphical illustration of the variation of the diffusioncoefficient between earth formation types using one embodiment accordingto the present disclosure;

FIG. 6 shows a graphical illustration of the variation of thermalneutron capture cross sections between earth formation types using oneembodiment according to the present disclosure;

FIG. 7 shows a graphical illustration of the variation of thermalneutron scattering cross sections between earth formation types usingone embodiment according to the present disclosure;

FIG. 8 shows a graphical illustration of the variation of short-to-longspace detector count ratios over time using one embodiment according tothe present disclosure;

FIG. 9 shows a graphical illustration of the variation of R_(max) valueswith the hydrogen index using one embodiment according to the presentdisclosure;

FIG. 10 shows a graphical illustration of the weighted time-averagedratios as a function of porosity using one embodiment according to thepresent disclosure;

FIG. 11 shows a flow chart of another estimation method for oneembodiment according to the present disclosure;

FIG. 12 shows a graphical illustration of an energy spectrum capturedwith two components using one embodiment according to the presentdisclosure;

FIG. 13A shows a graphical illustration the first of two radiationinformation components after deconvolution using one embodimentaccording to the present disclosure;

FIG. 13B shows a graphical illustration of the second of two radiationinformation components after deconvolution using one embodimentaccording to the present disclosure; and

FIG. 14 shows a graphical illustration of an aggregation of energyspectra from the same pulse cycle across a series of time intervalsusing one embodiment according to the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to estimating a parameter of interest ofan earth formation using a radiation source, particularly nuclearradiation. The earth formation may be exposed to radiation, and moreparticularly a neutron source. Downhole tools may include a radiationsource and one or more detectors. Herein, the radiation source mayinclude, but is not limited to, one or more of: (i) a neutron source,(ii) a gamma-ray source, and (iii) an x-ray source. The detectors may beused to detect radiation from the earth formation, though the detectorsare not limited to detecting radiation of the same type as emitted bythe radiation source. Detectors may have shielding to prevent thecounting of radiation from unintended sources.

During operation of the radiation source, radiation may be emitted fromthe source into the earth formation to be surveyed and interact with thenuclei of the atoms or atoms of the material of the earth formationresulting in various nuclear reactions and/or gamma ray reactions suchas Compton scattering and pair-production.

The detectors may estimate the radiation count returning from the earthformation. If multiple detectors are used, the detectors may be spacedin a substantially linear fashion relative to the radiation source. Iftwo detectors are used, there may be a short spaced (SS) detector and along spaced (LS) detector, wherein the detectors have differentdistances from the radiation source. The SS and LS detectors are notlimited to being placed on the same side of the radiation source as longas their spacing from the radiation source is different. Additionaldetectors may be used as long as their spacing differs from the spacingof the other detectors relative to the radiation source. One of the twodetectors may be a neutron detector, while the other detector may be aneutron detector or another type of radiation detector, such as, but notlimited to, a gamma-ray detector and an x-ray detector.

In one aspect, the radiation source may be controllable in that theradiation source may be turned “on” and “off” while in the wellbore, asopposed to a radiation source that is “on” continuously. Due to theintermittent nature of the radiation source, radiation from the sourcewill reach differently spaced detectors at different times. When theradiation source transmits a signal, such as a pulse, the resultingresponse from the earth formation may arrive at the respective detectorsat different times.

In some embodiments, a neutron detector may be used to acquire radiationinformation from a volume of interest in the earth formation. Theneutron detector may detect neutrons and gamma rays emitted by thevolume of interest. The radiation information may include a firstcomponent and a second component. The first component may be a neutroncount, and the second component may be a non-neutron count. The firstand second components may be detected simultaneously. An algorithm maybe used to deconvolve the radiation information into the first componentand the second component. The second component may be used to estimate aparameter of interest of the volume of interest. The second componentmay be used with the first component to provide two depths ofinvestigation. Since the two components may be detected simultaneouslyusing a single neutron detector, the radiation information may becollected over a short period of time, such as a single pulse cycle.Herein, a pulse cycle is defined as the period between the initiation ofa first neutron pulse by a neutron source and a second pulse, thus thepulse cycle includes the neutron pulse period and its associated decayperiod. In one embodiment, the pulse cycle is about 1000 microseconds(60 microsecond pulse period and 940 microsecond decay period).

In some embodiments, porosity and SIGMA for a formation may be estimatedfor the same time period using a single measurement. In anotherembodiment, the second component may be a gamma count, and the gammacount may be used to estimate gamma driven SIGMA measurements for thevolume of interest. Since the first component (neutron count) may beused to estimate neutron driven SIGMA measurements for the volume ofinterest, the use of both components may provide improved SIGMAestimation and reduction in SIGMA estimation errors. The use ofsimultaneous gamma driven and neutron driven SIGMA measurements may bebeneficial since the effectiveness of the type of SIGMA measurement usedmay vary depending on the structure and composition of the earthformation. In the earth formation, even if both types of SIGMAmeasurement are effective, the different types of SIGMA measurements mayhave varying degrees of effectiveness at different depths (i.e. neutrondriven SIGMA for short range, gamma driven SIGMA for long range).

FIG. 1 schematically illustrates a drilling system 10 having a downholetool 100 containing a radiation source 140 and associated detectors 120,130 according to one embodiment of the present disclosure. As shown, thesystem 10 may include a conventional derrick 160 erected on a derrickfloor 170. A carrier 110, which may be rigid or non-rigid, may beconfigured to convey the downhole tool 100 into wellbore 150 inproximity to the earth formation 180. The carrier 110 may be a drillstring, coiled tubing, a slickline, an e-line, a wireline, etc. Downholetool 100 may be coupled or combined with additional tools.

Detector 130 may be a short spaced detector, and detector 120 may be along spaced detector. Radiation shielding (not shown) may be locatedbetween radiation source 140 and the detectors 120, 130. Radiationshielding may include, but is not limited to, gamma-ray shielding andneutron shielding. Drilling fluid 190 may be present between the earthformation 180 and the downhole tool 100, such that emissions fromradiation source 140 may pass through drilling fluid 190 to reach earthformation 180 and radiation induced in the formation 180 may passthrough drilling fluid 190 to reach the detectors 120, 130.

In one embodiment, the downhole tool 100 may use a pulsed neutrongenerator emitting 14.2 MeV fast neutrons as its radiation source 140.The electronics (not shown) associated with the detectors may be capableof recording counts from at least two axially spaced detectors 120, 130with very narrow time bins (or windows) and generating a time-dependentratio between the at least two axially spaced detectors by usingtime-dependent information from multiple bursts. Herein, “information”may include raw data, processed data, and signals. The ratio, R, may betypically calculated by dividing the count rates from one detector bythe count rates from the second detector. This ratio may be expressed asa curve or other graphical function that describes a combination ofmultiple ratio values. In some embodiments, the parameter of interestmay be estimated using a difference between the detector counts. Herein,the term time-dependent broadly describes the property of the ratio tovary with time, as opposed to a ratio that remains constant, such aswith a continuous radiation source. In some embodiments, thetime-dependent ratio may be weighted by a function of the time. Forexample, the function may be an exponential function having timevariable. It is a general function of time that may take other forms aswell. The axially spaced detector count rate ratios may be obtained as afunction of time and graphically illustrated as a time-dependent ratiocurve. Various properties of the earth formation may be determined usingthe time-dependent ratio curve, including, but not limited to, porosity,sigma, diffusion correction, and hydrogen index of the earth formation.

As shown in FIG. 2, method 200 is a method for estimating at least oneparameter of interest using a time-dependent ratio based on informationacquired from at least one neutron detector and a second detectoraccording to the present disclosure. Method 200 may include step 210,where a radiation source emits a neutron pulse in proximity to an earthformation. In step 220, the resulting interactions between the neutronpulse and the material of the earth formation result in thermalizedneutrons, which may be detected by short spaced detector 130 and longspaced detector 120. Herein, the neutron interactions may include, butare not limited to, elastic scattering, inelastic scattering andcapture. In step 230, time-dependent ratios may be generated using theradiation information collected by the detectors. In the alternative,information collected by the detectors may be used to generate adifference between the radiation counts estimated by the detectors. Instep 240, a parameter of interest of the earth formation may beestimated using the time-dependent ratios. The estimation of theparameter of interest may also include comparison or combination of thetime-dependent ratios with reference information about the earthformation. In some embodiments, estimation method 200 may include step250, where reference information on the earth formation or earthformations generally is collected. Reference information may be combinedwith time-dependent ratios in step 240 to estimate a parameter ofinterest.

As shown in FIG. 3, certain embodiments of the present disclosure may beimplemented with a hardware environment that includes an informationprocessor 300, an information storage device 310, an input device 320,processor memory 330, and may include peripheral information storagemedium 340. The input device 320 may be any information reader or userinput device, such as data card reader, keyboard, USB port, etc. Theinformation storage device 310 stores information provided by thedetectors. Information storage device 310 may be any non-transitorymachine-readable information storage medium, such as a USB drive, memorystick, hard disk, removable RAM, EPROMs, EAROMs, flash memories andoptical disks or other commonly used memory storage system known to oneof ordinary skill in the art including Internet-based storage.Information storage medium 310 stores a program that when executedcauses information processor 300 to execute the disclosed method.Information storage medium 310 may also store the earth formationinformation provided by the user, or the earth formation information maybe stored in a peripheral information storage medium 340, which may beany standard computer information storage device, such as a USB drive,memory stick, hard disk, removable RAM, or other commonly used memorystorage system known to one of ordinary skill in the art includingInternet based storage. Information processor 300 may be any form ofcomputer or mathematical processing hardware, including Internet-basedhardware. When the program is loaded from information storage medium 310into processor memory 330 (e.g. computer RAM), the program, whenexecuted, causes information processor 300 to retrieve detectorinformation from either information storage medium 310 or peripheralinformation storage medium 340 and process the information to estimate aparameter of interest. Information processor 300 may be located on thesurface or downhole.

In one embodiment, the detectors 120, 130 predominantly count thermalneutrons released by the earth formation after exposure to a 40 μsecpulse. The use of a 40 μsec pulse duration is illustrative and exemplaryonly, as different pulse durations may be used. Typically, the pulsewill be “on” for a substantially shorter period than the pulse will be“off”. When the pulse is “off,” the radiation counts start to decay veryrapidly and soon reach an asymptotic decay rate. As shown in as shown inFIG. 4, the short spaced detector radiation count 410 and the longspaced detector radiation count 420 do not decay at the same rates. Thisdifference in decay rates results in short spaced/long spaced ratio 430.If the decay rates were the same, the ratio 430 would have been constantthrough the period. As shown in FIG. 4, the ratio varies significantlythrough the period.

In step 210, when a volume of interest in an earth formation is exposedto a neutron pulse, the neutrons in the volume of interest may beremoved by two primary mechanisms. The first one is neutron captureinteractions. The second one is escape of thermal neutrons from thevolume of interest. This may be termed a diffusion effect that isdescribing diffusion of neutrons from high neutron flux to low neutronflux areas. The neutron current term used in the neutron diffusiontheory can be used to provide insight into this mechanism. For aone-dimensional case in a Cartesian coordinate system, the net currentterm through an interface normal to x-direction is given as

$\begin{matrix}{J = {{- D}\frac{\mathbb{d}\phi}{\mathbb{d}x}}} & (1)\end{matrix}$

In this case, D represents the diffusion coefficient of the material and

$\frac{\mathbb{d}\phi}{\mathbb{d}x}$represents the spatial gradient of the neutron flux. The diffusioncoefficient may be estimated by total and scattering cross sections. Inthis case, these are the thermal energy range total and scattering crosssections. The following equation gives the expression for calculatingdiffusion coefficient.

$\begin{matrix}{D = \frac{1}{3\left( {\Sigma_{t} - {{\overset{\_}{\mu}}_{0}\Sigma_{s}}} \right)}} & (2)\end{matrix}$

In this equation, Σ_(t), Σ_(s) and μ ₀ may represent the total andscattering cross sections and average scattering angle cosine. As theexpression implies, the diffusion coefficient is a material propertybecause the total and scattering cross sections are material propertiesand they vary with the material composition. In FIG. 5, the diffusioncoefficients of three earth formation types are plotted as a function ofporosity. The porosity is saturated with fresh water. The capture andscattering cross sections driving the change in the diffusioncoefficients are shown in FIGS. 6 & 7.

As the equation (1) shows, net current uses a product of the diffusioncoefficient and the neutron flux gradient. Hence, the diffusion isdetermined by not only the material properties but also by the neutronflux gradient as well. In step 220, detectors 120, 130 count theradiation coming from the earth formation. For cases where the diffusioncoefficient is fixed, for the sake of simplicity, the larger differencebetween the short and the long spaced detector count rates may result inlarger current values.

In step 230, time-dependent ratios may be estimated for the countscollected by the detectors. Since the diffusion of neutrons from highflux volumes to low flux volumes varies the neutron gradient, the netcurrent term may change with time as a result of the variation in thespatial gradient. Since time variation of the counts is of interest, theratio will change with time as well. In other words, the neutrons willdiffuse from high flux areas to low flux areas and spatial gradient willdecrease resulting in the reduction in the net current term with thetime. As a result of this, a ratio of short spaced to long spaceddetector count rates will approach an asymptotic value. Regardless, theoverall process will manifest itself as a variation in the short-to-longdetector ratio in time.

If more than two detectors are used, multiple ratios may be taken fordetermining additional parameters. Depending on how the ratios areprocessed, multiple parameters may be determined from the samemultidimensional information. In step 240, the time-dependent ratios maybe used to estimate a parameter of interest. Illustrations of uses forthe time-dependent ratios to determine some parameters of interest aredetailed below.

Hydrogen Index Determination

An issue of interest with the variation of ratios is the impact of theearth formation type, porosity and formation fluids on the magnitude anddecay rates of the ratios. FIG. 8 shows ratios for various earthformation types with varying levels of porosity saturated with water.The solid line curves are for 10 pu formations of limestone 810,sandstone 820, and dolostone 830. The broken line curves are for 40 puformations of limestone 840, sandstone 850, and dolostone 860. A 40 puformation will have a higher hydrogen atom density present in the earthformation than a 10 pu formation. The maximum ratio values, R_(max) 870,observed will follow the hydrogen density in the earth formation. Thereason for this is the increase in the absolute hydrogen atom density inthe earth formation dominantly increases the thermalization rate andthus creates a larger spatial gradient between the short and long spaceddetectors. Therefore, R_(max) 870 may be related back to the amount ofhydrogen present in the earth formation. If formation porosity is filledwith water, oil or gas, then these substances will correlate tohydrogenous fluid filled porosity. FIG. 8 shows the R_(max) 870 valuesplotted against the porosity for the three earth formation types 810,820, and 830. When the porosity level is high, the ratios are higher ingeneral. The earth formation properties may impact the time-dependentratios, however, their effects may be small relative to the hydrogenousfluid filled porosity. The earth formation type impact can be observedby comparing, R_(max) values for sandstone, limestone and dolostone 870.Variation of R_(max) with hydrogen index and earth formation type can beseen in FIG. 9.

Sigma Determination

The sigma of the earth formation may be estimated using the timevariation of the count rates obtained through a single detector. In somecases, sigma may be estimated using gamma counts induced by neutrons,however, this example is illustrative and exemplary, as other counts,such as, neutron counts may be used as well. A set of time-dependentratio may be used to estimate the value of sigma. The ratio of the shortspaced and long spaced detector count rate or absolute or relativedifferences of count rates may be used to estimate sigma. The windowsize and position are apparatus dependent and are primarily optimizedbased upon minimizing measurement uncertainty, background noise, andexternal effects on the measurement of the parameter of interest. Thewindow averaged ratio R may be computed by averaging the R values overthe selected time window. The time window and width may be chosen eitherwith a static or a dynamic width. The location of the window on the timescale may be moved to capture the best position to be able to measurethe most accurate sigma value with lowest uncertainty possible. Variousstatistical approaches can be used to determine the proper location andwidth of the windows selected to compute R. R value computed through theselected approach is then compared to a calibration curve to extract thesigma for the measurement.

Diffusion Correction

Diffusion correction allows for correction of the detector counts due toerrors caused by neutron leakage from the volume of interest. Diffusionis driven by not only the material properties but by the spatialgradient of the neutron flux as well. In some embodiments, correctionmeasurement may take into account both material properties and thespatial gradient of neutron flux. This may be achieved by using multiplecorrelations. While R_(max) accounts for the neutron flux spatialgradient for the earth formation, the slope of R, R′, provides thesecond parameter needed for the second contribution. R′ can bedetermined by using either preset and pre-sized time windows or at achosen point in the time scale. Once the R_(max) is determined from loginformation, corresponding correlation is used with R′ to determine theamount of the correction to be applied to the sigma. R′ may be definedby standard calculus as

$\begin{matrix}{R^{\prime} = {{\lim\limits_{{\Delta\; t}->0}\frac{R_{1} - R_{2}}{t_{1} - t_{2}}} \approx \frac{\Delta\; R}{\Delta\; t}}} & (3)\end{matrix}$Porosity Determination

The magnitude of the ratio may be heavily affected by the porositybecause the pore volume contains hydrogen, oxygen and carbon and someother atoms. The variation of R will be different as well. Time weightedvalues of R, R _(t), may be used to obtain the porosity values from themeasurements. As with the previous estimates, the window size andlocation may be adjusted dynamically or statically. Statistical methodscan be used to determine window location and window width. One exampleof such a porosity determination is shown in FIG. 10, where four windowsizes have been used to obtain R _(t). The weighting scheme includesweighting the R values directly by time values. Other time, weightingalgorithms may be used to emphasize on different features of the Rvalues, without limit.

FIG. 11 shows one embodiment of a method 1100 for estimating at leastone parameter of interest of an earth formation using a first componentand a second component of an information set obtained using a singleradiation detector according to the present disclosure. Method 1100 mayinclude step 1110, where a radiation source emits a neutron pulse inproximity to an earth formation. In step 1120, the resultinginteractions between the emitted neutrons and the material in a volumeof interest of the earth formation results in the release of secondaryradiation, which may be detected by a detector (e.g., detector 120, ordetector 130). The radiation may include, but is not limited to, one ormore of: thermalized neutrons and gamma rays. In some embodiments, asingle detector (e.g., either of detector 120, or detector 130) may beused to detect the radiation. In some embodiments, the information setmay be obtained during a single pulse cycle. In step 1130, aninformation set representing the radiation received by the detector maybe deconvolved to separate the information set into a first componentand a second component. In step 1140, a parameter of interest of theformation may be estimated using, alone or in combination, one or moreof the first component and the second component. The first component mayinclude a particle count and the second component may include anelectromagnetic radiation count. The first component may include aneutron count and the second component may include a non-neutron count,such as a gamma ray count. In some embodiments, the first component mayinclude information about a first depth of investigation and the secondcomponent may include information about a second depth of investigation.The estimation of the parameter of interest may also include comparisonor combination of one or more of the components with referenceinformation about the earth formation. In some embodiments, estimationmethod 1100 may include step 1150, where reference information on theearth formation or earth formations generally is collected.

In step 1130, the information set may be deconvolved using techniquesknown to those of skill in the art with the benefit of the presentdisclosure. In one exemplary technique, the information set, which maybe expressed as N(x), may be divided by energy level into threesegments: (i) a first segment that may include radiation counts atenergy levels below a neutron energy range, (ii) a second segment thatmay include radiation counts at energy levels in the neutron energyrange, and (iii) a third segment that may include radiation counts atenergy levels above the neutron energy range. A new energy spectrum maybe formed by combining the first segment and the third segment.

Next, the new energy spectrum (the second component) may be modeled by afunction using the formula:N _(γ)(x)=ax ^(b)  (4)where N_(γ)(x) is a gamma count, x is the energy level, and a and b areconstants that may be estimated using regression.

The first component may be estimated by subtracting the second componentfrom the information set, as seen in the formula:N _(n)(x)=N(x)−N _(γ)(x)  (5)where N_(n) (x) may be a neutron count.

The total neutron and gamma counts may be obtained using the formulas:

$\begin{matrix}{N_{n} = {\int_{x_{\min}}^{x_{\max}}{{N_{n}(x)}\ {\mathbb{d}x}}}} & (6) \\{N_{\gamma} = {\int_{x_{\min}}^{x_{\max}}{{N_{\gamma}(x)}\ {\mathbb{d}x}}}} & (7)\end{matrix}$where x_(max) and x_(min) may be integration bounds for the energyspectrum.

FIG. 12 shows a typical energy spectrum recorded by a Li-6 neutrondetector over a time interval after a volume of interest has beenexposed to a neutron pulse. The energy spectrum 1210 may have a peak1220 due mainly to neutrons and a continuum 1230 due mainly to a gammarays. The use of a Li-6 neutron detector is exemplary and illustrativeonly, as other neutron detectors known to those of skill in the art withthe benefit of this disclosure may be used.

FIGS. 13A & 13B show the energy spectrum of FIG. 12 after deconvolutioninto a first component and a second component. FIG. 13A shows the secondcomponent 1310, which may be a non-neutron component. In this example,the second component is a gamma ray count. FIG. 13B shows the firstcomponent 1320, which may be a neutron component.

FIG. 14 shows a series of energy spectra captured over several timeintervals. In this example, each time interval (also known as a timebin) is about 10 microseconds in duration. Including a curve 1410representing a time interval (0-10 microseconds after the end of theneutron pulse) and curve 1420 representing another time interval(180-190 microseconds after the end of the neutron pulse), there are 19curves representing 19 time intervals.

While the foregoing disclosure is directed to the one mode embodimentsof the disclosure, various modifications will be apparent to thoseskilled in the art. It is intended that all variations be embraced bythe foregoing disclosure.

We claim:
 1. A method for estimating at least one parameter of interestof an earth formation using a downhole tool in a borehole in theformation, the method comprising: conveying the downhole tool in aborehole in the earth formation; controlling a radiation source on thedownhole tool to emit radiation from the source into the earthformation; collecting an information set from only a single radiationdetector on the downhole tool positioned such that the single radiationdetector is at a first distance from the radiation source, theinformation set representing radiation detected by the single radiationdetector responsive to the emitted radiation, wherein the informationset from only the single radiation detector comprises an energyspectrum; separating the information set into a first component and asecond component, wherein the first component is a particle countcomprising a neutron count and the second component is anelectromagnetic radiation count comprising a gamma ray count, by usingthe gamma ray count to derive the neutron count, comprising: dividingthe information set by energy level into three segments, including: (i)a first segment that includes radiation counts at energy levels below aneutron energy range, (ii) a second segment that includes radiationcounts at energy levels in the neutron energy range, and (iii) a thirdsegment that includes radiation counts at energy levels above theneutron energy range; combining the first segment and the third segmentto form a new energy spectrum; attributing the new energy spectrum tothe gamma ray count; and using the new energy spectrum from theinformation set to estimate the neutron count; and estimating the atleast one parameter of interest using the first component and the secondcomponent of the information set, wherein the second component isrelated to radiation induced in the earth formation.
 2. The method ofclaim 1, wherein the information set is obtained during a single pulsecycle.
 3. The method of claim 1, wherein the first component is from afirst radial depth of investigation and the second component is from asecond radial depth of investigation.
 4. The method of claim 1, whereinthe downhole tool comprises: a second radiation detector on the downholetool positioned at a second distance from the radiation source that isdifferent than the first distance; and wherein the method furthercomprises: collecting a second information set representing radiationdetected by only a second radiation detector; separating the secondinformation set into another first component and another secondcomponent, including deconvolving the information set into the anotherfirst component and the another second component; and estimating the atleast one parameter of interest using the another first component andthe another second component of the information set obtained using asingle radiation detector on a downhole tool.
 5. The method of claim 1,wherein the radiation comprises radiation from the formation responsiveto a radiation source on the tool at a first distance from the singleradiation detector, the method further comprising: emitting radiationfrom the radiation source at a first borehole depth; and detecting theradiation with the radiation detector at only a second borehole depth.6. An apparatus for estimating at least one parameter of interest of anearth formation, comprising: a downhole tool comprising: at least oneradiation detector on the downhole tool; a carrier configured toposition the single radiation detector in a borehole in the earthformation; and a radiation source on the downhole tool; at least oneprocessor; a storage device; and a program stored on the storage devicecomprising instructions that, when executed, cause the at least oneprocessor to perform a method, the method comprising: controlling theradiation source on the downhole tool to emit radiation from the sourceinto the earth formation; collecting an information set from only asingle radiation detector of the at least one radiation detector on thedownhole tool, the information set representing radiation detected bythe single radiation detector, the radiation source positioned such thatthe single radiation detector is at a first distance from the radiationsource, wherein the information set from only the single radiationdetector comprises an energy spectrum; separating the information setinto a first component and a second component, wherein the firstcomponent is a particle count comprising a neutron count and the secondcomponent is an electromagnetic radiation count comprising a gamma raycount, comprising deconvolving the energy spectrum into the neutroncount and the gamma ray count, comprising: dividing the information setby energy level into three segments, including: (i) a first segment thatincludes radiation counts at energy levels below a neutron energy range,(ii) a second segment that includes radiation counts at energy levels inthe neutron energy range, and (iii) a third segment that includesradiation counts at energy levels above the neutron energy range;combining the first segment and the third segment to form a new energyspectrum; attributing the new energy spectrum to the gamma ray count;and using the new energy spectrum from the information set to estimatethe neutron count; and estimating the at least one parameter of interestusing the first component and the second component of the informationset, wherein the second component is related to radiation induced in theearth formation.
 7. The apparatus of claim 6, wherein the informationset is obtain during a single pulse cycle.
 8. The apparatus of claim 6,wherein the first component is from a first radial depth ofinvestigation and the second component is from a second radial depth ofinvestigation.
 9. A method for estimating at least one parameter ofinterest of an earth formation using a downhole tool, comprising:controlling a radiation source on the downhole tool to emit radiationfrom the source into the earth formation; collecting an information setfrom a neutron detector on a downhole tool responsive to the emittedradiation, wherein the information set from the neutron detectorcomprises an energy spectrum; estimating the at least one parameter ofinterest using a non-neutron component of the information set obtainedusing the neutron detector by deconvolving the energy spectrum into theneutron component and the non-neutron component, comprising: dividingthe information set by energy level into three segments including: (i) afirst segment that includes radiation counts at energy levels below aneutron energy range, (ii) a second segment that includes radiationcounts at energy levels in the neutron energy range, and (iii) a thirdsegment that includes radiation counts at energy levels above theneutron energy range; combining the first segment and the third segmentto form a new energy spectrum; attributing the new energy spectrum tothe gamma ray count; and using the new energy spectrum from theinformation set to estimate the neutron count; wherein the non-neutroncomponent is related to radiation induced in the earth formation.